Facsimile machines allow users to send documents over familiar analog Plain Old Telephone Service ("POTS") designed for placing ordinary voice telephone calls. POTS is typically delivered over a subscriber loop of copper wires installed between each subscriber location, such as a home or office, and the local telephone company ("telco") central office. Over the pair of copper wires, facsimile data are transmitted between subscribers and the telco central office. The central office then provides circuit-switching equipment to establish connections between subscribers. In such a circuit-switched system, a circuit connection is established for each facsimile call and is maintained for the duration of the call.
Recently, on-line computer services, such as the Internet, have changed the way subscribers use their telephones. On-line computer services typically make a dial-up telephone number available for users with a modem to access the service. Many users of on-line computer services now spend several hours each day on the telephone connected to services such as the Internet. Such a usage pattern ties up the telephone for incoming calls and causes a strain for other members of the household who wish to place calls. In response, households have added a second telephone line for computer data traffic. To provide a second telephone line, a second pair of copper wires is usually provided between the subscriber's location and the telco central office. In addition to more copper wiring, a second telephone line also requires additional central office connection and transmission equipment.
Recently, higher capacity data transmission services have become available to carry a subscriber's computer data to remote computer systems. Such data transmission services often carry data on the same copper pair utilized by POTS. To allow simultaneous data and POTS service and provide greater bandwidth, higher capacity data services operate at frequencies above the 1 KHz to 4 KHz voice frequency band used by POTS. For example, such data services may operate in the frequency range around 80 KHz or higher.
The growing popularity of on-line computer services has also challenged the assumptions upon which the telephone network was constructed. The public switched telephone network (PSTN) is designed with the assumption that only about 10% of residential users and 20% of business users are using the telephone at any given time. The PSTN telephone line and associated circuit-switching equipment are thus designed to be shared by only the fraction of subscribers actually using the telephone at any time. Now, many households use the telephone for several hours each day to carry data traffic to computer services. Using a POTS circuit-switched telephone call to carry data traffic inefficiently consumes hardware resources, since a dedicated circuit connection is consumed for the entire duration of the call.
More recently, packet-switched data networks have been established to carry high-speed data traffic between distributed computer systems. In addition to providing higher data rates, packet-switched networks are more hardware efficient than circuit-switched networks for carrying data. A packet-switched network establishes a virtual circuit connection which uses transmission resources only when data is actually transmitted. Such a virtual connection is well suited for users of computer services who are connected for long periods of time and spend a relatively small proportion of time actually transmitting and receiving data.
Despite the change in communication needs and usage patterns, most data traffic from homes or small offices is carried by POTS voice lines.